perf: Reduce audio test durations for faster test suite

Optimized long-running audio tests to significantly improve test suite
performance while maintaining test coverage.

Changes:
- WavDumpBackend: Added set_duration() method with configurable duration
  - Default remains 60s for debugging/production use
  - Test now uses 2s instead of 60s (140x faster: 60s → 0.43s)

- JitteredAudioBackendTest: Reduced simulation durations
  - Test 1: 2.0s → 0.5s (4x faster)
  - Test 2: 10.0s → 3.0s (3.3x faster)
  - Overall: 14.49s → 4.48s (3.2x faster)

- Updated assertions for shorter durations
- Progress indicators adjusted for shorter tests

Results:
- Total test suite time: 18.31s → 8.29s (55% faster)
- All 20 tests still pass
- Tests still properly validate intended behavior

handoff(Claude): Optimized audio test performance to speed up development
iteration without sacrificing test coverage. WavDumpBackend now has
configurable duration via set_duration() method.

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